While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In their simplest forms, organic light-emitting diodes (OLEDs), also referred to as organic electroluminescent devices, contain spaced electrodes separated by an organic light-emitting structure which emits light in response to the application of an electrical potential difference across the electrodes. At least one of the electrodes is light-transmissive, and the organic light-emitting structure can have multiple layers of organic thin films which provide for hole injection and transport from an anode, and for electron injection and transport from a cathode, respectively, with light emission resulting from electron-hole recombination at or near the internal junction formed at the interface between the hole-transporting and the electron-transporting thin films.
The range of EL devices in recent years has expanded to include not only single color emitting devices, such as red, green and blue, but also devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International de l'Eclairage (CIE) 1931 chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color. Typical color combinations that produce white light are red, green and blue; blue-green and orange; blue and yellow; but others are known.
Notwithstanding this development there are continuing needs for organic EL device components, such as blue light emitting layers, that will provide even higher luminance efficiencies while maintaining low device drive voltages and long lifetimes combined with high color purity. Such improvements in blue light emitting layers are particularly beneficial for white-light-producing OLEDs since blue light emitters tend to have lower efficiencies than emitters of other colors, so that in order to produce balanced whites, the blue pixel (in a RGB OLED device) may need to be driven at higher current densities, resulting in increased power consumption as well as shortened blue lifetimes, and the color-complementary layer(s) (in a white OLED) may need to operate at less than their maximum potential efficiency.
Styrylamines (for example, see U.S. Pat. No. 5,121,029 and U.S. Pat. No. 7,544,425) are well known emitters of blue light. Anthracenes are well known as electron-transporting materials (for example, see U.S. Pat. No. 6,387,546) useful as hosts in light-emitting layers (for example, see U.S. Pat. No. 5,935,721) including those with styrylamine emitters (for examples, see U.S. Pat. No. 6,534,199, U.S. Pat. No. 7,252,893, U.S. Pat. No. 7,504,526 and U.S. Pat. No. 6,713,192).
US Patent Publication 2007/0252522 discloses OLED devices with a blue light-emitting layer (LEL) with a styrylamine emitter and an anthracene host where the electron-transporting layer (ETL), which is located in contact with the LEL, consists of an anthracene.
U.S. Pat. No. 7,074,500, US Patent Publications 2002/0197511 and US 2006/0251923 all disclose OLEDs with a thin electron-blocking layer between the LEL and the ETL. US Patent Publication 2007/0075631 discloses OLEDs with a thin electron-impeding layer between the LEL, which is preferably phosphorescent, and the ETL, where the materials in the thin layer can be hole-transporting and have a LUMO level that is less negative than the LUMO levels of the ETL and the host in the LEL. US Patent Publications 2003/0175553 and US 2003/0068528 both show the use of Ir(ppy)3 as a thin hole-blocking layer on the cathode side of a LEL.
U.S. Pat. No. 7,018,723 discloses OLEDs with ETLs that contain naphthalene and benzene derivatives. Lee et al., Applied Physics Letters, 92(20), 203305/1-3 (2008) discloses OLEDs with ETLs that contain silylated benzene derivatives.
However, these devices do not necessarily have all desired EL characteristics in terms of high luminance, low drive voltage, and sufficient operational stability. Notwithstanding all these developments, there remains a need to improve efficiency of blue light-emitting layers in OLED devices, as well as to provide embodiments with other improved features.